Diagnostic x-ray imaging devices when operated in a real time mode (e.g. fluoroscopy) require a system to maintain the x-ray exposure to the detector and the displayed image brightness at optimum levels. Such a system allows the brightness of an image to remain relatively constant despite the fact that the density and thickness of the tissues being imaged are varying. This system requires a brightness detector which senses the signal level in the image and uses this information to control the x-ray exposure and displayed brightness.
Conventional x-ray imaging systems based on image intensifier and video camera perform brightness detection either through use of a small area photosensor at the output of the image intensifier or through use of the analog video signal from the camera. The information from the brightness detector is fed back to the x-ray generator to adjust the x-ray tube voltage and current. Adjustment of x-ray tube voltage and current controls the x-ray exposure to the imager and normally maintains fairly constant brightness. To maintain the optimum brightness in circumstances where the desired exposure cannot be attained, however, a separate signal-level control, independent of the exposure, is required. In conventional systems, this additional control is provided by adjusting an aperture in the image gate and/or by changing the gain of the video camera.
Large area solid state detectors are currently being developed in the x-ray art. In these detectors, conventional analog brightness detection methods are not applicable. Unlike an image intensifier, there exists no minified light image in such a detector. Therefore, methods which employ a small area photosensor cannot be directly adapted. Either a large area sensor or a means of concentrating light from a large area into a small area would be required. Neither of these approaches is attractive from the standpoint of packaging and cost. Also, for a large area solid state imager, an analog signal equivalent to the video signal in an image intensifier/video camera system is not easily accessible. Instead, the easily-available signals are in digital form.
Since conventional brightness detection methods are inappropriate for a fully digital detector, it would be desirable to have a digital brightness detector with increased flexibility and additional features compared to conventional approaches. Such a detector would be beneficial in any system, including image-intensifier-based systems, in which the image is available in digital form.